Is the Universe Infinite or Finite? I was having that debate with a friend of mine today. My position was that it is finite because it is expanding. My logic was that the fact that is expanding indicates that it must be finite, because how could something be infinite and getting bigger?
His position was that it was infinite though he couldn't really explain why he thought that. (to give you some context, he also believes his daily horoscope offers him meaningful guidance and that aliens built the pyramids).
You may have guessed by now that neither one of us are physicists, so to settle the argument, I proposed that we contact an expert.
Confident of my correctness, I suggested to my buddy that we should have friend of this here site @seanmcarroll (the brilliant physicist and author of "From Eternity to Here") give us an answer.
I tweeted at Sean the subject line of this thread and he was kind enough to tweet me back in a matter of minutes. He said:
"Nobody knows. Sometimes that's the answer."
He then tweeted
"There may not even be an answer. de Sitter space (e.g.) can be finite or infinite depending on how you slice it."
So not wanting to badger Sean with follow up questions, I ask you: If we know the Universe is continually expanding (which to me suggests that it has a finite size that is increasing), how can it be labeled as anything other than finite?
A millennia ago, we thought the universe revolved around the Earth. Just a century ago, we thought the Milky Way was everything. Then we learned that the universe contains countless galaxies, not just the Milky Way. Now it appears quite possible that our universe is one of many -- perhaps even one of trillions of universes.
If the universe began with a bang and has been expanding ever since, then the only way it could be considered infinite is in its geometry -- its shape. Not that I really understand it but the universe might have a saddle shape that would make it virtually infinite. Regardless, the notion of a universe that is infinite in size always seemed less than satisfying to me. One can easily imagine standing outside the universe and looking at other universes in the distance, twinkling like little stars.
There are different ideas of what a multiverse might be like. They might be like other dimensions coexisting with ours but to which we have no access of any kind. One theory suggests the weakness of gravity might be explained by the physics of such a multiverse. I think the notion of multiverse that's gaining the most favor right now is one in which space is unimaginably larger that we ever thought it was and is populated by many universes -- even an infinite number of universes -- moving away from each other so fast that they leave light in their dust and, thus, are not visible to each other.
It appears that, so far, the universe is only as infinite as our imagination.